Another option would be... if you don't mind driving a manual tranny would be a ZF 5 speed out of a later model F250 or F350. The ZFs that came from behind a 351w will bolt right up behind your 300. I swapped one into my 96 F150 after destroying the POS M5OD. It was a direct bolt in swap, it used the same clutch, slave cyl, trans mount, and reverse light switch. I love the deeper first gear (5.72) for starting with a heavy load and towing, yet it still has the same .76 overdrive that the Mazda tranny had. Oh BTW please do post the info on which flat tops you're using I am very interested.